Step #1) Buy a floppy drive for $10
Step #2) Flash...
The thing is, the BIOS is what makes your mobo to start to tell what the other things do. If the file does not copy 100% correctly, and is not flashed 100% correctly, then you will have a paperweight. Some boards' bios chips are socketed now, but usually a bad flash equals garbage for motherboard.
You can get some windows programs to flash while in windows, which is like the easy way out, but be very very careful with those. Personally I wouldnt touch them with a 10m pole.
I consider myself rather computer literate, and 1/4 BIOS flashes have ended in failure. Its just environment and luck really once you get to it. And I wouldnt use a USB drive. Not for something this basic. Get a floppy. Get a used floppy. Be happy. Even XP requires a floppy to install SATA drivers remember and XP came out way after the turn of the millenium. That and floppy drives make that awesome "Grr-Drr-drr-Dunk" sound whenever the computer boots.